A stream of produced fluids containing hydrocarbon products produced from a subterranean reservoir contains several components that must be separated: a stabilized crude oil generally having a vapor pressure of 14.7 psia or less, condensate, liquefied petroleum gas (LPG) and methane. LPG refers to propane, butane and mixtures thereof. In addition to these components, other components that are frequently separated are ethane and water. Further, contaminants such as sulfur and other non-carbon and non-hydrogen elements may also be separated out of the crude oil and gases. A significant amount of capital must be spent for facilities to separate hydrocarbon containing produced fluids into these components.
When crude oil is produced in remote locations away from markets, either onshore or offshore, these products are typically stored in appropriate tanks, and then shipped via oceangoing vessel, pipeline, train or truck transportation. LPG is usually not sold as a mixture and almost all locations will separate the LPG into specification propane and butane thus adding to the expense of the separation, storage and transportation.
Condensate refers to a light hydrocarbon mixture that is separated from stabilized crude oil. It typically contains pentane, hexane, and can contain small amounts of butane. These more volatile condensates are often shipped separately from stabilized crude oil.
Typically, the amount of methane that is produced along with the crude oil is insufficient to justify conversion to Liquefied Natural Gas (LNG). But options to handle this methane (and ethane) are limited. Natural gas often cannot be burned (flared) as this will impact local regulations around greenhouse gas emissions. Also, natural gas often typically is not reinjected into a producing formation as this will dilute the crude oil and lead to a loss in crude oil production. Likewise often local uses, such as combustion of the natural gas for facilities uses, are insufficient to consume this gas.
One technology currently used to handle associated gas that cannot be flared, reinjected or used in local markets is to convert the associated gas into synthetic fuels such as diesel, jet fuel, and naphtha by a Fischer-Tropsch process. Conventional Fischer-Tropsch processes make a very waxy product that is subsequently converted into premium quality transportation fuels. Gas conversion to these products via the Fischer-Tropsch process is well known such as is described in U.S. Pat. No. 7,479,216.
The conventional Fischer-Tropsch conversion process is an expensive process and when used on associated gas, facilities distinct from those for crude oil must be used to handle the premium Fischer-Tropsch diesel, jet fuel, condensate and other products. Simply blending these products into the crude oil would result in a loss in their value. Likewise, the wax from the Fischer-Tropsch process has such a high melting point that the conventional Fischer-Tropsch product cannot be shipped in conventional crude tankers, but instead, requires expensive ships suitable for handling this high melting temperature material. As described in U.S. Pat. Appln. No. 2006/0069296, conventional crude tankers are often limited to material having pour points at or below 140° F. (60° C.).
Blending the wax from the Fischer-Tropsch process into crude oil is not an option either. Blending as little as 2 wt % Fischer-Tropsch product containing waxes into some crude oils may increase the pour point above 60° C. Also, conventional Fischer-Tropsch products will contain substantial quantities of olefins, alcohols and acids. When blended with crude oil these Fischer-Tropsch products can cause the crude oil to be difficult to refine and may lead to a discount in the crude sale price.
There is a need for a low-cost process to convert associated gas from a stream of produced fluids produced from a subterranean formation into a low-impurity synthetic crude oil while avoiding the difficulties caused by wax content. There is a further need for such a process to convert associated gas to a synthetic crude oil that can be blended at an amount greater than 2 wt % with natural crude oil, the natural crude oil being derived from the stream of produced fluids, wherein a blended stabilized crude oil having a pour point at or below 60° C. is produced.